Lean-burn engines are operated predominantly with a lean air/fuel mixture over their time of operation. Lean-burn engines include diesel engines and lean-burn spark-ignition engines.
To improve air quality, exhaust-gas legislation in Europe and in other industrial countries is being made ever more stringent. Following the ruling of the European parliament, the limit values of Euro 5 should become legally binding for new vehicles from Sep. 1, 2009 onwards, and the limit values of Euro 6 from Sep. 1, 2014 onwards. Euro 5 defines limit values for diesel vehicles of 180 mg/km for nitrogen oxides and 5 mg/km for soot particles. With Euro 6, the limit value for nitrogen oxides will be reduced to 80 mg/km. In California, a limit value of 50 mg NOx/mile and 10 mg soot/mile has been in force even since 2007.
The exhaust gas of lean-burn engines contains, as pollutants, unburned hydrocarbons, carbon monoxide, nitrogen oxides and soot. The planned limit values for the concentrations of nitrogen oxides and soot in the exhaust gas can no longer be achieved using engine-internal measures alone; exhaust-gas aftertreatment is in fact necessary for this purpose: the nitrogen oxides can be converted using nitrogen oxide storage catalytic converters or SCR catalytic converters, and the soot can be filtered out using a soot filter. A soot filter must be regenerated every 500 to 1000 km traveled by raising the exhaust-gas temperature to more than 550° C. and burning off the soot which has been deposited in the filter. However, the regeneration of the soot filter, which lasts approximately 15 minutes, makes it difficult to adhere to the nitrogen oxide limit values: the catalytic activity of the catalytic converters which are used for nitrogen oxide reduction is low at the high temperatures which occur during the filter regeneration. Furthermore, the high exhaust-gas temperatures damage the catalytic converters.
The present invention is concerned with the aftertreatment of exhaust gas in a lean-burn engine having catalytic converters for converting the nitrogen oxides and having a soot filter. The catalytic converters for converting the nitrogen oxides are also referred to below as denitrogenization catalytic converters.
DE 103 93 184 T5 (which corresponds to U.S. Pat. No. 7,055,311 B2) describes a twin-flow exhaust-gas purification system for a diesel engine having a plurality of cylinders. The exhaust-gas purification system comprises a first exhaust line for the exhaust gases of a first group of cylinders and a second exhaust line for the exhaust gases of a second group of cylinders. Arranged in each exhaust line is a nitrogen oxide storage catalytic converter and, if appropriate, a soot filter. The two exhaust lines are merged downstream of the storage catalytic converter at an opening-in point into a common exhaust line. The common exhaust line comprises an oxidation catalytic converter. The compositions of the exhaust gases in the first and second exhaust lines are set independently of one another by the electronic engine controller, such that the exhaust gas in the one line is enriched for the regeneration of the storage catalytic converter while the exhaust gas in the other line is lean. The enrichment and leaning are adjusted such that a lean exhaust gas is present in the common exhaust line downstream of the merging of the two exhaust-gas flows, and any possible leakage of the reducing agent is oxidized on the oxidation catalytic converter.
The inventors of the present patent application have established that the exhaust-gas purification system of DE 103 93 184 T5 is not capable, during the regeneration of the soot filter, of converting the nitrogen oxides which are emitted by the engine into non-harmful components.